Hallo everyone! The same problem, now in 2020 with UE MEGABOOM white 2018 model! In my situation the speaker suddenly STOPPED working and when I changed device (I use it in a Macmini and I make a try with an iPhone) it couldn’t play up to the third volume line (if the maximum of volume is 10, my speaker play maximum at 3).
After many many articles, I tried your suggestion:
I pressed the Volume Up and Down and the Bluetooth button at the same time WHILE the speaker was plugged in and it totally closed without coming back AND THEN I pressed the power button with the Volume Down button and everything became absolutely OK!
I hope we will have an Update… This problem scared me as !&&*!
Thank you everyone for your suggestions and help!